Recent evolution of optical communications networks emphasizes increased functionality of optical components, including wavelength filtering together with ADD/DROP capability and also power monitoring and adjustment for all the signal paths. Silica-on-silicon Planar Lightwave Circuits (PLC) is now a mature technology, producing highly stable and reliable optical components capable of accomplishing the above functions with excellent performance. The capability of integrating multiple functions in the same chip is highly desirable to reduce module size, simplify fiber management and reduce manufacturing cost.
PLC Mach-Zehnder Interferometer (MZI) devices have been proposed and investigated for applications as optical switches and variable optical attenuators. The principle of operation is based on the thermo-optic effect of silica; by producing heat with a thin film heater positioned above the waveguide the effective index in one of the MZ arms is increased in order to create a phase difference between the arms.
One of the main problems associated with silica-on-silicon MZI devices is the polarization dependence, which can limit the extinction ratio of the optical switches and result in large PDL in the region of high signal attenuation in the case of variable optical attenuator (VOA) devices. At the same time, the power consumption of the MZI devices on Si substrate tends to be large due to the much higher thermal conductivity of Si compared to the thermal conductivity of glass. Complete switching of light between zero and maximum attenuation requires about 0.5 W of power. The power consumption can be substantially reduced by etching trenches on each side of the MZ arms to confine the heat to a small region surrounding the waveguide.
The problem of polarization dependence in silica on silicon devices is addressed in prior U.S. Pat. No. 4,781,424 by Masao Kawachi et al. of Nippon Telegraph and Telephone Corporation. In this patent it is stated that, “tensile stresses are imparted to the inside of the film surface of the cladding layer because of the difference in thermal expansion coefficient between the cladding layer and the silica glass substrate. That is, it exhibits stress-induced birefringence”.
In one embodiment Kawachi et al. propose a stress relief groove or grooves formed in the cladding layer adjacent to the core portion in order to control birefringence of the optical waveguide. Grooves are formed by a reactive ion etching process. If desired after forming grooves through the cladding layer, grooves can be further recessed into the silicon substrate using a wet etching liquid (for instance, a mixture of hydrofluoric acid, nitric acid and acetic acid). It is taught that birefringence can be controlled by the position of the stress relieving grooves, and that in the regions where grooves are provided birefringence can be reduced to substantially zero. It is suggested that it is effective to position a short groove at a selected location to locally vary the birefringence characteristics in the optical waveguide, as an alternative to a uniform groove along the core portion of an optical waveguide.
Kawachi et al. disclose a further preferred embodiment comprising a Mach Zehnder interferometer in which stress relieving grooves are provided on one of the waveguide arms over a segment length ΔL corresponding to the difference in length between the two waveguide arms. In this stress groove region, periodic recess regions are etched underneath the waveguide. The device is described as polarization insensitive.
Undercut etched grooves in planar lightwave circuit devices are also disclosed in U.S. Pat. No. 6,031,957 by Ryoji Suzuki et al. of Hitachi Cable Ltd issued Feb. 29, 2000. In this case, the waveguide structure is continuously separated from the substrate in the longitudinal direction of the waveguide core by etching away a thin silicon film between the cladding material and the substrate. A series of intermittent lateral grooves permit introduction of the etching substance while leaving intermittent cladding material supporting the suspended waveguide structure above the substrate. This structure is said to provide good heat insulating structure for preventing heat dissipation to a surrounding portion in a lateral direction from the core.
In combination, the prior art seems to teach that a planar lightwave circuit having lateral grooves and a continuous undercut separating the waveguide from the substrate would provide both heat isolation and polarization insensitivity.
In investigating the prior art, this teaching was found to be insufficient. As will be discussed in detail later, it was found that a device with continuous lateral grooves or trenches does improve the heat isolation and thus reduce the switching power, but the polarization dependent loss is higher than without grooves or trenches. Furthermore, the additional process requirements for producing an undercut waveguide are costly and reduce the strength and integrity of the device.
It is desired to create a planar lightwave circuit attenuator or switch which can provide greater dynamic range or higher extinction ratio and improved polarization dependent loss, while reducing its power consumption.